This application's broad objective is to pursue studies establishing the pericyte as a morphogen-responsive osteoprogenitor cell. This pursuit is validated by evidence that the pericyte produces alkaline phosphatase (ALP) and osteocalcin in cell culture. Specific aims investigate both in vivo and in vitro responses of the pericyte to bone morphogen and/or bone hormone stimulation. In vivo bone production in implants of bovine brain-derived pericytes combined with an osteoconductive matrix is determined histologically in athymic nude mice. The morphogen specificity of this response is determined employing BMP-treated and non-treated pericytes. The presence of new bone formation on the osteoconductive matrix is determined by light microscopy on excised decalcified stained implants. Bovine pericyte origin of cells associated with new bone formation in the nude mouse is determined immunohistologically using a MHC I bovine monoclonal antibody for indirect immunohistofluorescent analysis. In vitro studies analyze the cyclic AMP and ALP responses of BMP-treated and non-treated pericyte cultures to parahormone (PTH) stimulation. Northern hybridization analysis is also employed on these cultures to determine PTH effects on procollagen and vitamin D receptor (VDR) mRNA's as well as 1,25(OH)2VitD3 effect on the VDR transcript. Additionally the temporal relationships of transcripts associated with the evolving osteoblast phenotype are analyzed in both BMP-treated and non-treated pericyte cultures. The temporal appearances of cfos, TGF-beta, procollagen type I, ALP, osteocalcin and VDR transcripts are determined by Northern hybridization analysis. The existence of an inducible osteoprogenitor cell is supported by endochondral bone formation at extraosseous implantation sites of demineralized bone matrix or purified morphogen; the identity of this cell is still speculative. Identifying this cell would broaden our ability to study osteogenesis from its inception and, hence, further understand and possibly treat failures of bone repair (non unions) and bone renewal processes (osteoporoses). The pericyte's candidacy in assuming a pivotal role in osteogenic repair/renewal processes is pursued by showing its direct association with in vivo, extraosseous, morphogen-induced bone formation; by showing patterns of biochemical responses to bone hormones similar to those classically associated with the osteoblast; and elucidating a temporal profile of mRNA's coding factors important to the evolving osteogenic phenotype. The last is the basis for future studies clarifying molecular controls of this evolving phenotype.